Four-wave-mixing based optical wavelength converter device

ABSTRACT

A wavelength converter device is provided for generating a converted radiation at frequency ω g  through interaction between at least one signal radiation at frequency ω s  and at least one pump radiation at frequency ω p , including an input for the at least one signal radiation at frequency ω s , a pump light source for generating the at least one pump radiation at frequency ω p , an output for taking out the converted radiation at frequency ω g , a structure for transmitting the signal radiation, including two optical resonators having a non-linear material, having an optical length of at least 40*λ/2, λ being the wavelength of the pump radiation, and resonating at the pump, signal and converted frequencies ω p , ω s  and ω g , wherein by propagating through the structure, the pump and signal radiation generate the converted radiation by non-linear interaction within the optical resonators.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national phase application based onPCT/EP2002/007207, filed Jun. 28, 2002, the content of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength converter devicecomprising a structure having a plurality of cascaded opticalresonators.

The present invention relates, moreover, to an use of a structurecomprising a plurality of cascaded optical resonators for generating aradiation at frequency ω_(g) through non-linear interaction between atleast one pump radiation at frequency ω_(p) and at least one signalradiation at frequency ω_(s).

Furthermore, the present invention relates to an use of a structure,comprising a plurality of cascaded optical resonators made of anon-linear material, for altering the optical spectrum of at least onesignal radiation at frequency ω_(s) by non-linear interaction within thematerial of the optical resonators.

Moreover, the present invention relates to an apparatus for an opticalnetwork node comprising a routing element and a wavelength converterdevice of the invention.

Additionally, the present invention relates to an optical communicationline comprising an optical transmission path for transmitting at leastone signal radiation at frequency ω_(s) and a wavelength converterdevice of the invention.

2. Description of the Related Art

Structures made of a plurality of cascaded optical resonators are known.

For example, A. Melloni et al. (“Synthesis of direct-coupled-resonatorsbandpass filters for WDM systems”, Journal of Lightwave Technology, Vol.20, No. 2, February 2002, pages 296-303) disclose a structure consistingof cascaded direct-coupled ring resonators or cascaded Fabry-Pérotresonators for use as a bandpass filter.

Furthermore, U.S. Pat. No. 5,311,605 discloses an optical devicecomprising a length of optical waveguide having incorporated therein anextended sequence of coupled single-resonator structures for use as anoptical slow wave structure. This document states that the structure mayalso be designed to provide a desired filter characteristic, adispersion such as to correct for undesirable dispersion in othercomponents of an optical system or to provide pulse expansion orcompression. The Applicant notes that no mention of use of non-linearinteractions is made in this document.

In a WDM (wavelength division multiplexing) optical communicationsystem/network, wavelength management and wavelength routing controlbetween nodes of the system is crucial for preventing wavelengthblocking and facilitating cross connecting. To this end wavelengthconverter devices able to shift a signal radiation from an opticalchannel to another are required.

Non-linear wavelength converter devices using a parametric process areknown.

A parametric process is a process typical of materials having anon-linearity of the χ₂ or χ₃ type according to which electromagneticradiation at predetermined frequencies that propagate in such materialsinteract with each other for generating electromagnetic radiation atdifferent frequencies from those that have generated them.

For example, a parametric process is a process according to which a pumpradiation at frequency ω_(p) that propagates in a non-linear material,interacting with a signal radiation at frequency ω_(s), generates aradiation at frequency ω_(g).

Typical parametric processes are a difference frequency generationprocess, according to which ω_(g)=ω_(p)−ω_(s), a sum frequencygeneration process, according to which ω_(g)=ω_(p)+ω_(s), a second orthird harmonic generation process, according to which ω_(g)=2ω_(p) or,respectively, ω_(g)=3ω_(p) and a degenerate four-wave mixing (FWM)process according to which ω_(g)=2ω_(p)−ω_(s) or ω_(g)=2ω_(p)+ω_(s).

For example, one-dimensional-photonic-crystal multiresonator structures(also called photonic band-gap structures) have been proposed forwavelength conversion through a second harmonic generation parametricprocess.

A one-dimensional photonic crystal structure typically consists of aperiodical alternation of two layers of material having differentrefractive indexes. The multiple reflections at the interfaces betweenthe two layers at different refractive index generate constructive anddestructive interference between the transmitted light and the reflectedlight, so that the propagation of electromagnetic waves in the photoniccrystal structure is allowed in some intervals of frequencies (orwavelengths) and forbidden in other intervals. The layers typically havethicknesses a and b of λ/4n (quarter-wave layer) or λ/2n (half-wavelayer), where λ is the operating wavelength and n the refractive indexof the layer, so as to form a periodic quarter-wave, half-wave or mixedquarter-half-wave structure.

WO 99/52015 describes a second harmonic generator based on a periodicphotonic crystal structure. The described structure comprises aplurality of layers of a first and a second material that periodicallyalternate, and has a band edge at the pump radiation frequency and asecond transmission resonance near the band edge of the second orderband gap at the generated second harmonic frequency. The layers havethicknesses a and b of λ/4n or λ/2n.

U.S. Pat. No. 6,002,522 discloses a structure having materials withdifferent refractive indexes and periodically arranged to form aphotonic band-gap structure. Furthermore, it discloses to set the periodof two different materials having different refractive indexes (that isthe thickness of a pair of two different materials) at nearly half thewavelength of light used. This document teaches that the structure canbe used to manufacture a wavelength converter by second harmonicgeneration, if a second-order non-linear optical material is used, andan optical switch if a third-order non-linear optical material is used.

Wavelength converter devices using the four-wave mixing process areknown.

EP 0 981 189 discloses a non-linear wavelength converter devicecomprising an optical waveguide doped with a rare earth element. Aninput optical signal and at least one pump light cause four wave mixing(FWM) to occur in the optical waveguide and the FWM causes a convertedoptical signal to be produced in the optical waveguide. The opticalsignal and the pump light are amplified in the optical waveguide therebythe four-wave mixing converted optical power is increased.

In order to increase the four-wave mixing converted optical power, alsowavelength converter devices using the four-wave mixing process in asingle optical resonator have been disclosed.

P. P. Absil et al. (“Wavelength conversion in GaAs micro-ringresonators”, Optics Letters, Vol. 25, No. 8, Apr. 15, 2000, pages554-556) disclose a device comprising a single micro-ring resonatorwherein a pump wave of frequency ω_(p) and a signal wave of frequencyω_(s) are launched into the ring at two different resonant frequencies.A new converted wave is generated by degenerate FWM at the frequencyω_(g)=2ω_(p)−ω_(s). The Authors states that non-linear interactions areenhanced in the resonator.

U.S. Pat. No. 5,243,610 disclose a device comprising an input for aninput light signal, an optical source for generating a pump lightsignal, a non-linear optical medium for receiving the pump and inputlight signals and an output. The non-linear optical medium includes aFabry-Perot type semiconductor laser and frequency converts the inputlight signal to generate an output light signal using non-degeneratefour-wave mixing. In this document it is stated that the FWM may begenerated at a relatively lower power due to an internal electric fieldenlarged by confining the pump light and input signal light into aresonator.

U.S. Pat. No. 5,550,671 discloses a device comprising an input for asignal radiation, an optical source for generating a pump radiation, alaser cavity and an output. The laser cavity is composed of a rare-earthdoped fiber and is defined by a pair of fiber Bragg gratings. Bypropagating through the laser cavity the signal and pump radiationgenerate by four wave mixing a new converted signal of wavelength within10% of the signal radiation wavelength. In this document it is statedthat the device can be made in compact form with a cavity length assmall as 100 m and can provide inverted signals at the same intensity asthe input signals.

The Applicant notes that in the above mentioned devices with a singleoptical resonator, the four-wave mixing converted optical power (i.e.the optical power of the radiation generated by four-wave mixing)depends on the pump power, on the resonator physical length and on thepower reflectivity of the reflectors forming the resonator.

For cost, availability and reliability reasons, the pump power should bekept as low as possible. Therefore, the resonator physical length andthe power reflectivity should be kept as high as possible in order toachieve high converted optical power values.

However, in this regard the Applicant notes that also the frequencydifference between two consecutive resonant frequencies (free spectralrange or FSR) and the bandwidth B of an optical resonator depend on thephysical length and on the power reflectivity. Furthermore, for use in aWDM optical communication system, the FSR and bandwidth B of theresonator should be set according to the WDM system requirements (e.g.,the bandwidth B′ of the WDM optical signals and the wavelength spacingthereof which is typically selected according to ITU-T recommendations).

It follows that, the physical length and the power reflectivity of theoptical resonator should be selected according to the WDM opticalcommunication system requirements and cannot be freely set to anydesired value.

Accordingly, the Applicant notes that external factors may not allowdesired values of the FWM converted optical power to be achieved. Thus,the above mentioned devices, using the four-wave mixing process in asingle optical resonator, are not versatile.

The Applicant has thus faced the technical problem of providing anefficient and versatile wavelength converter device, capable ofachieving high converted optical power values and, at the same time,meeting WDM optical communication system requirements.

SUMMARY OF THE INVENTION

It is a first aspect of the present invention a wavelength converterdevice, for generating a converted radiation at frequency ω_(g) byinteraction between at least one pump radiation at frequency ω_(p) andat least one signal radiation at frequency ω_(s), comprising

-   -   an input for said at least one signal radiation at frequency        ω_(s);    -   a pump light source for generating said at least one pump        radiation at frequency ω_(p);    -   an output for taking out said converted radiation at frequency        ω_(g);    -   a structure for transmitting said signal and pump radiation,        said structure including one optical resonator comprising a        non-linear material, having an optical length of at least        40*λ/2, wherein λ is the wavelength of the pump radiation, and        resonating at the pump, signal and converted frequencies ω_(p),        ω_(s) and ω_(g),        characterized in that said structure comprises a further optical        resonator coupled in series to said optical resonator, said        further optical resonator comprising a non-linear material,        having an optical length of at least 40*λ/2, wherein λ is the        wavelength of the pump radiation, and resonating at the pump,        signal and converted frequencies ω_(p), ω_(s) and ω_(g); wherein        by propagating through said structure the pump and signal        radiation generate said converted radiation by non-linear        interaction within said optical resonators.

As disclosed in more detail hereinafter, in the device of the inventionthe converted optical power depends on the resonator optical length, theresonator power reflectivity and the number of cascaded opticalresonators. Therefore, even if the resonator optical length and theresonator power reflectivity are constrained by external factors, thedesired converted optical power value can still be achieved by suitablyselecting the number of cascaded optical resonators.

Thus, the device of the invention can be suitably designed both toachieve high converted optical power values and to meet WDM opticalcommunication system requirements.

In the present description and claims, the expression “resonator” isused for indicating a device with a bounded path of such dimension thata standing electromagnetic wave can be sustained by application ofenergy of appropriate frequency. Typical examples of an opticalresonator are a guiding medium (conventionally named “resonant cavity”or “cavity”) bounded by two cascaded partially reflecting mirrors or aclosed-ring optical waveguide (conventionally named “microring”) with acoupling portion to allow the electromagnetic radiation to enter andexit from the ring. The optical resonator has a comb of resonantfrequencies that are substantially equispaced in frequency. The distancebetween two adjacent resonant frequencies is named free spectral range(FSR). The FSR depends on the group length L_(g) of the resonator(FSR=c/L_(g)). L_(g) is defined as L_(g)=c*τ_(g), wherein c is the speedof light and τ_(g) is the group delay of the resonator which depends onthe type of the resonator and on the material. In the case of a guidingmedium bounded by two cascaded partial reflectors, non-dispersive mediumand concentrated reflectors, L_(g) is twice the optical distance betweenthe two reflectors. In the case of distributed reflectors L_(g) can benumerically calculated by techniques well known in the art as, forexample, by means of the “coupled mode theory” (see for example S.Legoubin et al., “Free spectral range variations of grating-basedFabry-Perot filters photowritten in optical fibers”, J. Optical Societyof America, August 1995, Vol. 12, No. 8, pages 1687-1694). Lastly, inthe case of a closed-ring optical waveguide in a non-dispersive mediumL_(g) is the optical length of the ring.

Moreover, in the present description and claims, the expression

-   -   “optical length” for a radiation propagating in a propagation        medium is used for indicating the product between the refractive        index of the medium and the physical length thereof;    -   “four wave mixing efficiency” is used for indicating the ratio        P_(c)/(P_(p) ²*P_(s)) where P_(c) is the converted radiation        optical power, P_(p) the pump radiation optical power and P_(s)        the signal radiation optical power;    -   “bandwidth” B for an optical resonator is used for indicating        the full width at half maximum (FWHM) of each resonance;    -   “power reflectivity” or “reflectivity” is used for indicating        either the ratio between the power of the radiation reflected by        a partially reflecting mirror of a resonant cavity and the power        of the incident radiation, or the ratio between the power of the        radiation not coupled outside by the coupling portion and the        power of the incident radiation in a closed-ring optical        waveguide resonator;    -   “reflector” is used for indicating an element adapted to form a        resonator as, for example, a mirror of a resonant cavity, a        coupling portion of a closed-ring optical waveguide resonator or        a defect in a photonic crystal waveguide cavity;    -   “partial reflector” is used for indicating a reflector having a        power reflectivity lower than 100%;    -   “multiresonator structure” is used for indicating a structure        comprising a plurality of cascaded optical resonators;    -   “multistage device” is used for indicating a device comprising a        plurality of cascaded structures with phase-mismatch        compensating elements interposed between one structure and the        other;    -   “non-linear material” is used for indicating a material having        at least one of the χ₂, χ₃ susceptibility coefficients greater        than zero.

Furthermore, in a WDM optical communication system, the signal radiationhaving different wavelengths are each assigned a specific band ofwavelengths having predetermined width—hereinafter called “channel”.Each of said channels is characterised by a central wavelength value andby a range of wavelength, centred about said central wavelength, whichis defined “signal bandwidth or band” B′.

The dependent claims set out particular embodiments of the invention.

The non-linear material of the optical resonators can be of the χ₂ type.Advantageously, it is of the χ₃ type. In this latter case, the convertedradiation is preferably generated by four-wave-mixing. In a preferredembodiment of the invention, the four-wave-mixing is of the degeneratetype.

In a preferred embodiment of the invention, the pump radiation frequencyω_(p) and the signal radiation frequency ω_(s) are different.

Advantageously, the optical resonator and the further optical resonatorare directly connected in series.

Advantageously, the optical resonator and the further optical resonatorare made of the same material. This facilitates the manufacturingprocess of the structure.

Advantageously the optical resonator and the further optical resonatorare made of a transparent material at the working wavelengths (orfrequencies) of the device.

For example, the working wavelengths are selected within the intervalcomprised between 0.7 μm and 1.9 μm. According to a variant, they areselected within the interval comprised between 0.7 μm and 1.8 μm.Preferably, the working wavelengths are greater than 1.2 μm, morepreferably greater than 1.4 μm. Typically, they are lower than 1.7 μm.

Advantageously, the optical resonator and the further optical resonatorhave the same optical length.

Preferably, the optical resonator and the further optical resonatorresonate at three different resonant frequencies substantially equal tothe pump, signal and generated frequencies ω_(p), ω_(s), ω_(g),respectively.

According to another embodiment of the invention, the pump, signal andgenerated frequencies ω_(p), ω_(s), ω_(g) fall within the same resonantfrequency of the optical resonator and the further optical resonator.

Advantageously, the optical resonator and the further optical resonatoreach have a free spectral range equal to or lower than 4 THz. In fact,higher values of the free spectral range would imply higher values offrequency spacing ΔF between the pump and signal radiation and, thus, anappreciable decrease of the FWM converted optical power due to chromaticdispersion. Preferably, the optical resonator and the further opticalresonator each have a free spectral range equal to or lower than 1000GHz. More, preferably, the optical resonator and the further opticalresonator each have a free spectral range equal to or lower than 500GHz. According to a variant, the optical resonator and the furtheroptical resonator each have a free spectral range equal to or lower than100 GHz. According to another variant, the optical resonator and thefurther optical resonator each have a free spectral range equal to orlower than 50 GHz. According to another variant, the optical resonatorand the further optical resonator each have a free spectral range equalto or lower than 25 GHz.

The frequency spacing ΔF between the pump and signal radiation issubstantially equal to the free spectral range of the optical resonatorsor is an integer multiple thereof.

Advantageously, the optical resonator and the further optical resonatoreach have a bandwidth B of at least 100 MHz. Preferably, the bandwidth Bis of at least 1 GHz. More preferably, the bandwidth B is of at least2.5 GHz. More preferably, the bandwidth B is of at least 10 GHz. Evenmore preferably, the bandwidth B is of at least 20 GHz. Even morepreferably, the bandwidth B is of at least 40 GHz. Even more preferably,the bandwidth B is of at least 100 GHz. The above mentioned values allowthe device of the invention to be used with typical telecommunicationsoptical signal radiation modulated at 100 Mbit/s, 1, 2.5, 10, 20 and 40Gbit/s. Preferably, the bandwidth B of the optical resonator and of thefurther optical resonator is greater than the bandwidth of the opticalsignal radiation. More preferably, the bandwidth B of the opticalresonator and of the further optical resonator is at least twice thebandwidth of the optical signal radiation.

The optical signal radiation may be continuous wave (CW) signals or maybe modulated in amplitude, intensity, phase, frequency, polarization,according to any conventional technique, with any conventional format(e.g. NRZ, RZ, CRZ, soliton, duobinary).

Preferably, the optical resonator and the further optical resonatorcomprise reflectors having a power reflectivity of at least 0.5.Advantageously, the optical resonator and the further optical resonatorcomprise reflectors having a power reflectivity lower than or equal to0.9997 (corresponding to a transmissivity of −35 dB).

Preferably, the reflectors comprised in the optical resonator have thesame power reflectivity of the reflectors comprised in the furtheroptical resonator.

Advantageously, the ratio FSR/B for the optical resonator and thefurther optical resonator is at least equal to 2.

Advantageously, the ratio FSR/B for the optical resonator and thefurther optical resonator is lower than or equal to 100.

Preferably, the optical resonator and the further optical resonator eachhave an optical length lower than or equal to 7500*λ/2. Values ofoptical length higher than 7500*λ/2 would involve too low FSR andbandwidth B values (see Eq. 2b below). On the contrary, values ofoptical length lower than 40*λ/2 would involve too high FSR values and,consequently, too high frequency spacing ΔF values between the pump andsignal radiation and an appreciable decrease of the FWM convertedoptical power, due to chromatic dispersion.

According to a first embodiment of the invention, the optical resonatoris a Fabry-Perot like cavity bounded by two partially reflectingmirrors. Preferably, the further optical resonator is a Fabry-Perot likecavity bounded by two partially reflecting mirrors.

According to an embodiment, the partially reflecting mirrors areconcentrated. According to another embodiment, they are distributed.

Advantageously, the Fabry-Perot like cavity is formed in a rare-earthdoped bulk medium or a rare-earth doped waveguide or optical fiber.

According to a second embodiment of the invention, the optical resonatoris a microring like resonator. Preferably, the further optical resonatoris a microring like resonator.

According to a third embodiment of the invention, the optical resonatoris formed in a photonic crystal waveguide. Preferably, the furtheroptical resonator is formed in a photonic crystal waveguide.

Advantageously, the optical resonator and the further optical resonatorcomprise partial reflectors each having a uniform power reflectivity inthe whole wavelength range of interest (for example, at the pump, signaland converted radiation frequencies).

Preferably, the optical power of the converted radiation is of at least100 nW.

Advantageously, the optical power of the pump radiation is of at least100 mW.

Advantageously, the optical power of the signal radiation is of at least1 mW.

The non-linear material of the optical resonators is advantageouslyselected from the group comprising SiO₂, TeO₂, Al_(x)(GaAs)_(1-x),LiNbO₃, Si, InP, polymers, such as for example, PPV[poly(phenylene-vinylene)] or MEH-PPV (poly[2-methoxy,5-(2′-ethyl-hexyloxy)-p-phenylene-vinylene]), and combinations thereof.

The structure preferably further comprises a third optical resonatorcascaded to the further optical resonator. As to the features of thethird optical resonator, reference is made to what disclosed above withreference to the optical resonator and to the further optical resonator.

Preferably, the number of optical resonators cascaded in the structureis lower than N_(max), where N_(max) is equal to the ratio between thecoherence length L_(coh) of the structure and the physical length d ofeach optical resonator (see Eq. 7 below).

Typically, the output of the device comprises an optical filter coupledat the exit of the structure, adapted to take out the radiationgenerated at frequency ω_(g) from the device and to suppress a possibleresidual pump radiation at frequency ω_(p) and a possible residualsignal radiation at frequency ω_(s).

According to an embodiment of the invention the wavelength converterdevice comprises a further structure in series to the structure. In thisembodiment, the device preferably comprises also a phase mismatchcompensating element adapted to compensate for the phase mismatchaccumulated by the pump and signal radiation along the structure. Saidphase mismatch compensating element is advantageously placed between thestructure and the further structure.

As to the features of the further structure and the optical resonatorstherein comprised, reference is made to what disclosed above.

Preferably, the structure and the further structures have the samenumber of cascaded optical resonators. Furthermore, the opticalresonators of the structure are preferably the same as the opticalresonators of the further structure.

Advantageously, the structure and the further structures aresubstantially equal.

Advantageously, the phase mismatch compensating element comprises amaterial having a non-linear refractive index n2 lower than thenon-linear refractive index of the material included in the structureand the further structure.

Preferably, the second order dispersion coefficients β₂ and {circumflexover (β)} at the pump radiation frequency of the materials included inthe structures and in the phase mismatch compensating element haveopposite sign.

According to another embodiment, the second order dispersioncoefficients β₂ and {circumflex over (β)} at the pump radiationfrequency of the materials included in the structures and in, the phasemismatch compensating element have the same sign.

The phase mismatch compensating element can comprise either an opticaldielectric waveguide, a dispersive plate or an optical fiber.

According to another aspect, the present invention relates to a use of astructure comprising a plurality of cascaded optical resonators forgenerating a radiation at frequency ω_(g) through non-linear interactionof at least one pump radiation at frequency ω_(p) with at least onesignal radiation at frequency ω_(s), wherein said resonators comprise anon-linear material, resonate at the pump, signal and convertedfrequencies ω_(p), ω_(s) and ω_(g), and have an optical length of atleast 40*λ/2, wherein λ is the wavelength of the pump radiation.

Preferably, the radiation at frequency ω_(g) is generated by four-wavemixing. More preferably, by degenerate four-wave-mixing.

As to the features of the structure and the optical resonators referenceis made to what disclosed above with reference to the wavelengthconverter device of the invention.

According to a further aspect, the present invention relates to a use ofa structure, comprising a plurality of cascaded optical resonatorscomprising a non-linear material, for altering the optical spectrum ofat least one signal radiation at frequency ω_(s) propagating through it,by non-linear interaction of the optical signal radiation within thematerial of the optical resonators, wherein said optical resonatorsresonate at the signal radiation frequency ω_(s) and have an opticallength of at least 40*λ/2, wherein λ is the wavelength of the signalradiation.

Preferably, the optical spectrum is altered by using self-phasemodulation non-linear phenomenon.

Advantageously, the optical spectrum is altered through interaction withat least one pump radiation at frequency ω_(p) by using cross-phasemodulation non-linear phenomenon. In this case, the optical resonatorspreferably also resonate at the pump radiation frequency ω_(p).

As to the features of the structure and the optical resonators referenceis made to what disclosed above with reference to the wavelengthconverter device of the invention.

In a further aspect thereof, the present invention relates to anapparatus for an optical network node comprising

-   -   a routing element with at least one input port and a plurality        of output ports for interconnecting each input port with at        least one corresponding output port;    -   at least one wavelength converter device according to the        invention optically coupled to one of the ports of said routing        element.

As to the structural and functional features of the wavelength converterdevice, reference shall be made to what described above.

Typically, the apparatus also comprises at least one 1×K1 demultiplexerdevice. Said 1×K1 demultiplexer device is typically optically coupled toK1 input ports of the routing element.

Typically, the apparatus also comprises at least one K2×1 multiplexerdevice. Said K2×1 multiplexer device is typically optically coupled toK2 output ports of the routing element.

Advantageously, the routing element is selected from the groupcomprising the following elements: add-drop, cross-connect, λ-router(i.e. wavelength selective router) and switch, and a combinationthereof.

Advantageously, the apparatus also comprises N input optical fibers(with N≧1). In one embodiment, the N input optical fibers are opticallycoupled to N respective 1×K1 demultiplexer devices.

Advantageously, the apparatus also comprises M output optical fibers(with M≧1 and equal to or different from N). In one embodiment, the Moutput optical fibers are optically coupled to M respective K2×1multiplexer devices.

In a further aspect thereof, the present invention relates to an opticalcommunication line comprising an optical transmission path fortransmitting at least one signal radiation at frequency ω_(s) and awavelength converter device according to the invention, wherein saidwavelength converter device is optically coupled to said opticaltransmission path and generates a radiation at frequency ω_(g) bynon-linear interaction between at least one pump radiation at frequencyω_(p) and said at least one signal radiation at frequency ω_(s).

As regards the structural and functional features of the wavelengthconverter device, reference shall be made to what described above.

Advantageously, the optical transmission path is an optical fibrelength.

Typically, said optical communication line further comprises atransmitting station for providing said at least one signal radiation ata frequency ω_(s).

Advantageously, the transmitting station is adapted to provide aplurality n of optical signals having frequencies ω_(s1), ω_(s2) . . .ω_(sn) differing from one another. Preferably, the transmitting stationcomprises a wavelength multiplexing device for wavelength multiplexingthe plurality n of optical signals into a single WDM optical signal andfor sending said WDM optical signal along the optical communicationline.

Typically, said optical communication line further comprises a receivingstation connected to said optical communication line.

The receiving station advantageously comprises a wavelengthdemultiplexer device adapted to demultiplex a WDM optical signal comingfrom the optical communication line. Furthermore, the receiving stationis typically adapted to provide the demultiplexed signals to optionalfurther processing stages.

According to an embodiment, the optical communication line comprises anoptical node comprising an apparatus according to the invention, whereinsaid wavelength converter device of the line is comprised in theapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will appearmore clearly from the following detailed description of a preferredembodiment, made with reference to the attached drawings. In suchdrawings,

FIGS. 1 a and 1 b show a schematic view of a wavelength converter deviceaccording to first and second embodiment of the invention;

FIGS. 2 a and 2 b show a diagram illustrating the principle of FWMwavelength conversion according to first and second embodiment of theinvention;

FIG. 3 shows a schematic view of an ideal infinite periodic Fabry-Perotlike multiresonator structure;

FIG. 4 shows a plot of the ratio B/FSR versus the power reflectivity ofthe mirrors for the structure of FIG. 3 (FIG. 4 a) and a plot of theratio v_(g)/v_(f) versus the power reflectivity of the mirrors for thestructure of FIG. 3 (FIG. 4 b);

FIG. 5 is a plot of the conversion gain versus the power reflectivity ofthe mirrors for the structure of FIG. 3;

FIG. 6 shows a plot of the maximum converted optical power, which can beachieved using a single stage device of the invention, for differentnon-linear media;

FIG. 7 shows a schematic view of a Fabry-Perot like multiresonatorstructure according to a first embodiment of the invention;

FIG. 8 shows three different ways to spatially modulate the refractiveindex of an optical waveguide in order to form distributed partiallyreflecting mirrors;

FIG. 9 shows a schematic view of a microring like multiresonatorstructure according to a further embodiment of the invention;

FIG. 10 shows a schematic view of a photonic band gap like structureaccording to a further embodiment of the invention;

FIG. 11 shows a schematic view of a Fabry-Perot like structure in whichan active medium is used according to a further embodiment of theinvention;

FIG. 12 shows a plot of the maximum converted optical power, which canbe achieved using a multistage device of the invention, for differentnon-linear media;

FIG. 13 shows a schematic view of a multistage device according to anembodiment of the invention;

FIG. 14 shows a schematic view of a multistage device according to afurther embodiment of the invention;

FIG. 15 shows a schematic view of a multistage device according to afurther embodiment of the invention;

FIG. 16 shows a schematic view of a Fabry-Perot like multiresonatorstructure according to a further embodiment of the invention;

FIG. 17 shows the characteristic transmission profile for the structureof FIG. 16 (FIG. 17 a) and the plot of the ratio v_(g)/v_(f) versusf/FSR for the structure of FIG. 16 (FIG. 17 b);

FIG. 18 shows a schematic view of an optical communication lineaccording to an embodiment of the invention;

FIG. 19 shows a schematic view of an optical communication systemaccording to an embodiment of the invention;

FIG. 20 shows an apparatus for an optical network node according to anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a shows a wavelength converter device 100 comprising an input 1,an output 2, a pump light source 3 and a multiresonator structure 4including a plurality of optical resonators (for simplicity, only 2resonators 10, 20 are shown in FIG. 1 a) directly connected in series.

Advantageously, the wavelength converter device 100 further comprises anoptical isolator (not shown) for eliminating any back reflectedradiation exiting the device 100 through the input 1. Typically, theoptical isolator is placed between the input 1 and the first opticalresonator 10.

Advantageously, the wavelength converter device 100 comprises an opticalamplifier. Typically, the amplifier is placed at the output 2. Forexample, the optical amplifier is an erbium-doped fiber amplifier.

The input 1 is adapted to receive at least one signal radiation atfrequency ω_(s).

The pump light source 3 is adapted to generate at least one pumpradiation at frequency ω_(p).

Furthermore, the pump light source 3 is coupled to the structure 4through a conventional optical coupler 6. Preferably, the opticalcoupler 6 is a wavelength selective coupler.

The structure 4 is adapted to receive said signal and pump radiation.

The optical resonators 10, 20 are made of a non-linear material, eachhave at least two resonant frequencies substantially equal to the pumpand signal frequencies ω_(p) and ω_(s) and an optical length higher thanat least 40*λ/2, wherein λ is the wavelength of the pump radiation.

By propagating through the structure 4, the pump and signal radiationcause the generation of a converted radiation at frequency ω_(g)exploiting the non linear properties of the material of the opticalresonators.

The output 2 is adapted to take out the converted radiation at frequencyω_(g).

In a preferred embodiment of the invention, the material of the opticalresonators has a non-linearity of the χ₃ type. Moreover, the signal andpump frequencies ω_(s) and ω_(p) are different and the radiation atfrequency ω_(g) is generated using the degenerate four-wave mixing (FWM)process.

FIG. 2 a schematically shows the degenerate four-wave mixing processω_(g)=2ω_(p)−ω_(s), wherein ω_(s)=ω_(p)−nΔω, n is an integer number andΔω is the free spectral range of the optical resonators.

The applicant notes that, signal resonance 152 is not necessarily thefirst resonant frequency close to pump resonance 151, but can be shiftedof an integer number n of free spectral ranges; signal resonance 152 maybe at a lower frequency than pump resonance 151 or at a higher frequencyas well and, according to FWM properties, the converted radiation is thephase conjugated of the signal radiation and falls into the symmetricalresonance mode 153 with respect to the pump resonance 151. Furthermore,according to the principles of FWM wavelength conversion, in case ofmodulated signal radiation, the same modulation is transferred to theconverted radiation.

In FIG. 2 a the pump radiation frequency ω_(p) and the signal radiationfrequency ω_(s) are tuned to two different resonant frequencies of theoptical resonators 10, 20. However, as shown in FIG. 2 b, they may alsofall within the same resonant frequency 154 of the optical resonators10, 20, according to another embodiment of the invention.

The Applicant notes that an important requirement for the device of theinvention is that the optical resonators 10, 20 both resonate at thepump, signal and converted frequencies. However, it is not necessarythat they have the same finesse F (F=FSR/B). As shown below (see Eq. 2),this means that the optical resonators 10, 20 may have different valuesof power reflectivity R and optical length. However, in a preferredembodiment they have the same optical length. In a further preferredembodiment the reflectors included in the optical resonators 10, 20 havethe same power reflectivity.

Some basic notions for degenerate FWM are now briefly illustrated.

For a non-resonant waveguide structure and in the hypothesis of slowlyvarying envelope, undepleted pump, negligible self phase modulation,lossless media, the spatial evolution of the optical power P_(c) of theFWM converted radiation satisfies the following equation:

$\begin{matrix}{{P_{c}(z)} = {\gamma^{2}P_{p}^{2}P_{s}z^{2}\sin\;{c^{2}\left( \frac{\Delta\;{k \cdot z}}{2} \right)}}} & (1)\end{matrix}$where z is the spatial coordinate, P_(p) is the optical pump power andP_(s) is the optical signal power. γ is a coefficient that depends onthe non-linear refractive index n₂ and on the waveguide effective areaA_(eff), while Δk=2k_(p)−k_(s)−k_(c) takes into account the impact onfrequency conversion efficiency of the phase mismatch due to thedifferent wave vectors of interacting fields. Since γ is typically asmall number (about 1.5·10⁻³ m⁻¹W⁻¹ in a silica fiber) high pump power(about 1 W) and extremely long device (of the order of Km) are generallyrequired.

From this point of view the Applicant notes that the use of an opticalresonator for carrying out a FWM process leads to a double advantage.

First of all, a radiation, whose frequency coincides with a resonantfrequency of the optical resonator, spends much more time through theresonator of physical length L_(f) than through a non-resonant waveguidestructure of the same length. This effect derives from the fact that thegroup velocity v_(in) within the optical resonator is lower than thegroup velocity v_(out) within a non-resonant waveguide structure. As aconsequence the interaction time between an optical pump radiation and asignal radiation is considerably increased when both optical radiationare suitably tuned near a resonant frequency. The more the propagationis slowed (v_(in)<v_(out)) the longer the interaction time within theresonator. Since FWM is an interaction time dependent process, theconversion efficiency in an optical resonator is higher than in anequivalent non-resonant waveguide structure of the same physical lengthL_(f).

Moreover, within the optical resonator, a radiation whose frequencycoincides with a resonant frequency of the optical resonator, has apower P_(int) strongly enhanced with respect to the power P_(out) withina non-resonant waveguide structure. This effect too is due to the abovementioned slowed propagation group velocity and the more the propagationis slowed, the higher P_(int) compared to P_(out). Since FWM is anintensity dependent process, the conversion efficiency in an opticalresonator is further improved compared to that of an equivalentnon-resonant waveguide structure of the same physical length L_(f).

The FWM process in an ideal infinite periodic structure made of cascadedoptical resonators is now described.

As also described hereinafter in more detail, the cascaded opticalresonators can be made from a series of directly coupled Fabry-Perotlike cavities, optical microrings or a photonic band gap (PBG) waveguidewith proper defects therein.

In the following description the main topics of an ideal infiniteFabry-Perot like multiresonator structure is described, even if the sameteachings can be applied to optical microring and PBG structures.

FIG. 3 shows said multiresonator structure wherein partially reflectingmirrors 72 are placed at a distance d from each other into a substrate71 of refractive index n.

This multiresonator structure has a periodic spectral response. Only aradiation whose spectrum lies inside a pass-band resonance of thestructure can propagate; otherwise the radiation is backward reflected.

Important parameters of this kind of multiresonator structure have beentheoretically determined by the Applicant.

The bandwidth B of such structure is given by

$\begin{matrix}{B = {\frac{2{FSR}}{\pi}{arc}\;{\sin(t)}}} & (2)\end{matrix}$where t is the field transmission coefficient of each mirror 72 andFSR=c/2nd (Eq. 2b) is the free spectral range, i.e. the frequencydifference between two consecutive resonant frequencies of themultiresonator structure. In Eq. 2b, nd is the optical length of eachoptical resonator.

Thus, the selective behaviour of the structure, expressed by the finesseF=FSR/B, only depends on the power reflectivity R of the mirrors 72(where R=1−t²).

As shown in FIG. 4 a, the ratio between the bandwidth B and the FSR,i.e. the inverse of the finesse F, decreases versus an increasing mirrorpower reflectivity R.

Since intra-resonator propagation constant β is a function of ω, themultiresonator structure introduces a periodic frequency dispersion. Thegroup velocity, defined as dω/dβ, is found to be at each resonantfrequency

$\begin{matrix}{{v_{g}\left( f_{0} \right)} = {{\frac{c}{n}t} = {v_{f}t}}} & (3)\end{matrix}$where v_(f)=c/n is the phase velocity of the optical field in theintra-resonator medium and f₀ is the resonant frequency of thestructure. As the transmission coefficient is always less than unity,the group velocity within the structure is always lower than the phasevelocity.

In FIG. 4 b the ratio between the reduced intra-resonator group velocityv_(g)(ω₀) and the phase velocity v_(f) is plotted versus the powerreflectivity R of mirrors 72.

As shown in FIG. 4 b, the ratio v_(g)(ω₀)/v_(f) decreases versus anincreasing mirror power reflectivity R.

Therefore, high reflectivity mirrors 72 may practically reduce up tozero the group velocity of a propagating pulse (thereby increasing theconversion efficiency), even though this effect involves a bandwidth Breduction.

The Applicant found that the above mentioned results still hold for afinite quasi periodic multiresonator structure, when a sufficientlylarge number of resonators are employed (e.g. 5 resonators). However,the advantages of the invention stand up also for a structure with onlytwo cascaded resonators (see FIG. 6).

A good estimate for the conversion gain G (that is, the ratio betweenthe FWM converted optical power P_(c,mrs) in a multiresonator structureand the FWM converted optical power P_(c) in a non-resonant waveguidestructure using the same pump and signal radiation and the same nonlinear medium) of the multiresonator structure of FIG. 3 is given by

$\begin{matrix}{G \cong \left( \frac{1 + t^{2}}{2t^{2}} \right)^{2}} & (4)\end{matrix}$where t is the field transmission coefficient of each mirror.

FIG. 5 shows the behaviour of the conversion gain G versus the mirrorpower reflectivity R. An extremely high conversion enhancement can beachieved using high reflectivity mirrors.

Taking into account Eq. (1), the FWM converted optical power P_(c,mrs)for an ideal infinite multiresonator structure is given by

$\begin{matrix}{P_{c,{mrs}} = {\gamma^{2}P_{p}^{2}P_{s}z^{2}\sin\;{c^{2}\left( \frac{\Delta\;{\beta \cdot z}}{2} \right)}G}} & (5)\end{matrix}$where z is the spatial coordinate and Δβ=2β_(p)−β_(s)−β_(c) is theintra-resonator phase mismatch.

Eq. (4) and (5) hold in the above stated hypothesis of slowly varyingfield envelope, undepleted pump, negligible self phase modulation andlossless media.

The maximal length of the multiresonator structure is limited by thepresence of the phase mismatch Δβ between interacting fields. The phasemismatch Δβ derives from chromatic dispersion, waveguide dispersion, nonlinear cross phase modulation and self phase modulation. The coherencelength of the structure, that is the length after which power starts toturn back from the converted radiation to the pump radiation, is definedas

$\begin{matrix}{L_{coh} = {\frac{\pi}{{\Delta\;\beta}} = {\frac{\pi}{{\Delta\; k}}t}}} & (6)\end{matrix}$where Δk=2k_(p)−k_(s)−k_(c) is the phase mismatch term in a non-resonantwaveguide structure. Thus, the coherence length in the multiresonatorstructure is reduced with respect to a non-resonant waveguide structure.

Since highly non linear materials often show a highly dispersivebehaviour, chromatic dispersion is typically the dominant factor whichlimits the coherence length. Non linear cross phase and self phasemodulation should be contemplated in a high power conversion process orwhen chromatic dispersion is very low.

The maximum physical length L_(max) of the multiresonator structure ispreferably lower than or equal to the coherence length L_(coh).Therefore, the maximum number of resonators that can be cascaded to formthe multiresonator structure preferably is

$\begin{matrix}{N_{\max} = {\frac{L_{\max}}{d} = {\frac{L_{coh}}{d} = {\frac{\pi\; t}{{{\Delta\; k}}d}.}}}} & (7)\end{matrix}$

According to Eq. (2) the physical length d of each resonator may bewritten as

$\begin{matrix}{d = {\frac{c}{2n\;{FSR}} = {\frac{c}{n}\frac{{arc}\;{\sin(t)}}{\pi\; B}}}} & (8)\end{matrix}$and substituting Eq. (8) into Eq. (7) N_(max) is given by

$\begin{matrix}{N_{\max} = {\frac{\pi^{2}{nB}}{{{\Delta\; k}}c}.}} & (9)\end{matrix}$where the approximation arcsin(t)=t used in deriving Eq. (9) holds forhighly reflective mirrors.

The chromatic dispersion term Δk can be expressed in terms of thefrequency detuning ΔF (that is, the frequency spacing |f_(s)−f_(p)|)between the signal radiation and the pump radiation asΔk=β₂(2πΔF)²  (10)where β₂ is the second order dispersion coefficient at the pumpradiation frequency. Introducing the expression of Δk, Eq. (9) becomes

$\begin{matrix}{N_{\max} = {\frac{nB}{4c{\beta_{2}}\left( {\Delta\; F} \right)^{2}}.}} & (11)\end{matrix}$

Equation 11 relates the maximum number of cavity N_(max) to thebandwidth B, the frequency detuning ΔF and the material dispersion β₂.

The ratio between the maximum converted output power P_(c,mrs) and inputpowers P_(p) and P_(s) (that is, the maximum FWM efficiency) may bederived from Eq. (4), (5) and (6). Substituting into Eq. (5) thecoherence length L_(coh) from Eq. (6), said ratio is given by

$\begin{matrix}{\frac{P_{c,{mrs}}}{P_{p}^{2}P_{s}} = {\left( \frac{2\gamma}{\Delta\; k} \right)^{2}\left( \frac{1 + t^{2}}{2t} \right)^{2}}} & (12)\end{matrix}$

The number of cavities necessary to obtain such a power level is justequal to N_(max).

The Applicant notes that the first factor uniquely depends on non-linearand dispersive material properties while the second factor is given bythe multiresonator structure.

To sum up:

-   a) the bandwidth B and the FSR of the multiresonator structure    decrease versus an increasing physical length d of each resonator    [see Eq. 2 and Eq. 2b (FSR=c/2nd)];-   b) the inverse of the finesse F and the bandwidth B of the    multiresonator structure decrease versus an increasing mirror power    reflectivity R (see Eq. 2 and FIG. 4 a);-   c) the conversion gain G increases with an increasing mirror power    reflectivity R (see Eq. 4 and FIG. 5);-   d) the FWM converted optical power P_(c,mrs) in the multiresonator    structure increases with an increasing gain G and an increasing    distance z (see Eq. 5);-   e) for a multiresonator structure, the spatial coordinate z is equal    to N*d, wherein N is the number of cascaded resonators.

Following what said above, while designing a wavelength converter deviceof the invention, the power reflectivity R and physical length d of eachresonator can be selected according to the desired values for finesse F(B/FSR) and bandwidth B of the structure (that is, according to WDMoptical communication system requirements, e.g. the bandwidth of thesignals and the channel spacing) while the number of cavities N can beselected so as to obtain the desired value for FWM converted opticalpower P_(c,mrs).

Therefore, in the device of the invention, even if external factorsconstrain the selection of the power reflectivity R and the physicallength d of each resonator, desired values of converted optical powerand conversion efficiency can still be obtained by suitably selectingthe number N of cavities (with the only restriction that N is preferablyless than N_(max).)

Accordingly, with respect to a conventional wavelength converter deviceusing the four-wave mixing process in a single optical resonator, themultiresonator structure of the invention has an extra degree of freedom(i.e. the number N of resonators) which allows both an efficient andflexible wavelength converter device to be obtained.

FIG. 6 shows the maximum converted optical power P_(c,mrs) versus thenumber N of resonators when different non-linear materials (SiO₂, TeO₂and Al_(0.2)GaAs_(0.8)) are used to perform FWM wavelength conversion.

The curves of FIG. 6 were obtained through a computer simulation basedon Eq. 5 for a bandwidth B of 20 GHz, a frequency detuning ΔF of 2 THz,200 GHz spaced WDM channels, an effective area A_(eff) of the waveguideof 10 μm² and pump and signal radiation powers respectively of 100 mWand 10 mW.

For a AlGaAs multiresonator structure the number of cascaded resonatorsis preferably less than 11. As shown in FIG. 6, a higher number ofresonators may lead to a decrease of the FWM converted optical power,due to the high second order dispersion coefficient β₂˜1240 ps²/Km. ATeO₂ (β₂˜52 ps²/Km) multiresonator structure preferably has less than170 cascaded resonators and a SiO₂ (β₂˜−25 ps²/Km) multiresonatorstructure preferably has less than about 240 cascaded resonators.

It is worthwhile noting that AlGaAs has a non-linear refractive index n2more than ten times higher than TeO₂ Hence, a slightly inferiorconverted optical power can be obtained using an extremely low number ofresonators.

Resuming the description of the device of FIG. 1, the pump light source3 is a conventional laser or light emitting diode source.

Advantageously, the output 2 comprises an optical filter (not shown)suitable to let the converted radiation at frequency ω_(g) exit from thedevice 100 and to suppress any residual pump and any residual signalradiation.

When the pump and signal radiation propagate through the structure 4 inthe opposite direction, the device 100 preferably comprises also anoptical isolator (not shown), placed between the input 1 and the firstoptical resonator 10, to suppress the counter-propagating residual pumpwhile any residual signal radiation is suppressed by the filter.

The input 1 and output 2 are each advantageously connected to a fibrepigtail for facilitating the connection of the device 100 to otheroptical fibre elements.

The non-linear material of the optical resonators 10, 20 can be forexample selected from the group comprising SiO₂, TeO₂,Al_(0.2)GaAs_(0.8), polymers (for example, PPV), Si and InP.

The structure 4 can be made from either a waveguide or a bulkmultilayer.

Possible waveguides are dielectric waveguides (buried, rib, ridge ordiffused), photonic band gap (PBG) waveguides or optical fibers.

The cascaded optical resonators 10, 20 can be made from cascadedFabry-Perot cavities, cascaded optical microring resonators or properdefects in a photonic band gap waveguide.

FIG. 16 shows an example of a finite multiresonator structure 4 made ofeight cascaded optical resonators.

In said structure 4, nine partially reflecting mirrors 92 are insertedinto a waveguide integrated in a substrate 91 of refractive index n at adistance d from each other. The relatively high impedance of thesequence of resonators is preferably matched to the input and output ofthe device 100 by introducing at the beginning and at the end of thestructure 4 sections having mirrors 92 a, 92 b with lower reflectivity.

The suitable power reflectivity profile of the mirrors can be obtainedthrough known methods of synthesis of selective bandpass filters basedon directly coupled resonator.

For example, using the synthesis technique disclosed in the abovementioned article by A. Melloni et al. (“Synthesis of direct-coupledresonators bandpass filters for WDM systems”, Journal of LightwaveTechnol., Vol. 20, No. 2, February 2002, pages 296-303), the followingpower reflectivity profile can be obtained for an eight resonatorChebyshev like structure with a finesse F equal to 5. The powerreflectivity R of the internal mirrors 92 c is 0.88 while the powerreflectivity R of the first 92 a and the second 92 b mirrors (as well asthe power reflectivity R of the last two mirrors 92 a and 92 b) isrespectively 0.28 and 0.765 for the above mentioned impedance matchingrequirements. As this reflectivity profile performs a 20 dB in bandreturn loss (defined as the ratio between the back reflected opticalpower and the input optical power), signals tuned closely to a resonantfrequency may propagate without noticeably attenuation. Out of bandsignals are completely reflected because of a more than 60 dB stop bandrejection.

FIG. 17 a shows the power transmittivity (defined as the ratio betweenthe output optical power and the input optical power) versus f/FSR(wherein f is the frequency), for the structure of FIG. 16.

FIG. 17 b shows the ratio between the phase velocity and the groupvelocity versus f/FSR, for the structure of FIG. 16. As shown in FIG. 17b, in band signals are delayed nearly three times compared to signalspropagating in a non-resonant structure, in good agreement withtheoretical results. As a result a signal whose frequency lies near aresonant frequency ω_(o) spends more time travelling through themultiresonator structure 4 than it takes travelling through anequivalent non-resonant waveguide structure of the same physical lengthL_(f). Thus, when the FWM wavelength conversion takes place inside themultiresonator structure 4, the interaction time is increased as well asthe propagation velocity is reduced. Such enhancement is about 1/t,where t is the inner mirror field transmission coefficient.

FIG. 7 shows a Fabry-Perot like multiresonator structure 4, according toan embodiment of the invention, comprising an optical waveguide 120wherein a series of equi-spaced partially reflecting mirrors 124 areinserted in order to form a plurality of cascaded optical Fabry-Perotlike cavities. The optical length Lo of each cavity (Lo=d*n_(eff),wherein d is the distance between two consecutive mirrors and n_(eff) isthe effective refractive index of the material of which the waveguide120 is made) is of at least 40*λ/2, wherein λ is the wavelength of thepump radiation.

The partially reflecting mirrors 124 can be either concentrated (thatis, much shorter than the wavelength in the medium) or distributed (thatis, several quarter of wavelengths long). The concentrated reflector canbe obtained by means of an abrupt discontinuity between two materialswith a strong refractive index difference (for example, metal-glass).The distributed reflector can be achieved by a 1D, 2D or 3D periodicalstructure. For example, the 1D structure can be made of a conventionalBragg grating.

As well known in the art, forward and backward waveguide-mode-couplingare induced in a Bragg grating by periodically varying the effectiverefractive index n_(eff). Said periodical variation may be achieved byspatially modulating the n_(eff). For example, a lateral corrugation(FIG. 8 a) or an upper corrugation (FIG. 8 b)—induced by periodicallyetching the optical waveguide spatial profile—can be used. In fact, suchperiodical change in the waveguide width leads to a periodiclongitudinal modulation of the n_(eff). Otherwise a direct periodicalvariation of n_(eff) can be induced by the exposure of a photosensitivemedium in a pattern of diffracted UV light (FIG. 8 c). The structures ofFIG. 8 may be suitably combined and other methods to perform periodicalvariation of n_(eff) may also be used.

The Bragg gratings may be of the chirped type. This allows a broaderreflectivity band to be achieved.

The Bragg gratings may be of the apodized type.

As already disclosed above, wavelength conversion may be performed usingdegenerate four wave mixing process in the non-linear medium of thewaveguide 120. Input into the structure 4 are the pump radiation atfrequency ω_(p) and the signal radiation at frequency ω_(s). The pumpfrequency ω_(p) and the signal frequency ω_(s) are tuned as close aspossible to a resonance mode of the cascaded optical resonators,preferably as close as possible to the center of the resonance mode. Asthe radiation propagates inside the non-linear medium, a convertedradiation is generated within the structure 4 by FWM between the pumpradiation and the signal radiation. The frequency of the convertedoptical radiation is ω_(c)=2ω_(p)+ω_(s) or ω_(c)=2ω_(p)−ω_(s). Then, thepump, signal and converted radiation leave the structure 4 as shown inFIG. 7.

In FIG. 7 wherein the pump and signal radiation travel in the samedirection, forward FWM is performed. However, in general, backward FWMcan also be performed, wherein the pump and signal radiation propagatethrough the structure 4 in the opposite direction.

In one particular embodiment of the structure 4 of FIG. 7, the structure4 comprises eleven semiconductor Fabry-Perot like resonators obtained byinserting in a Al_(0.2)GaAs_(0.8) waveguide twelve equi-spaced partiallyreflecting mirrors.

The seven inner mirrors have a power reflectivity of about 0.97, thefirst and the last impedance matching mirrors have a power reflectivityof about 0.55 while the second and the last but one impedance matchingmirrors have a power reflectivity of about 0.94. This mirrorreflectivity profile performs a Chebyshev-like multiresonator structurewith a bandwidth B of 20 GHz and FRS of 200 GHz. Due to phase mismatch,the maximum frequency detuning Δf is preferably of 2 THz. The waveguidehas an effective refractive index n_(eff) of 3.3. Each cavity has aphysical length d of 217 μm and, neglecting reflector thickness, thewhole structure has a physical length of about 2.39 nm. The waveguidehas an effective area A_(eff) of 10 μm². A continuous 100 mW pumpradiation is tuned at one resonant mode of the structure (f_(p)=196THz). A modulated 10 mW signal radiation has a frequency f_(s) tuned atanother resonant mode of the structure (f_(s)=194 THz) and is 2 THzshifted with respect to the pump radiation, according to the case shownin FIG. 2 a. The pump and signal radiation co-propagate through thestructure 4 and both enter the device 100 from the input 1. A thirdconverted wave is generated by FWM at frequency f_(c) (f_(c)=198 THz).No significant power is backward reflected thanks to a 20 dB in bandreturn loss of the structure. The converted optical power P_(c,mrs) is5.9 μW (about −22 dBm, corresponding to the point A of FIG. 6). By usingEq. 4 and 5, more than 22 dB gain G is obtained compared to a 2.39 mmnon-resonant waveguide structure.

According to further embodiments of the present invention, in thestructure 4 a different number of cavities as well as differentsubstrates may be used.

FIG. 9 shows another embodiment of the invention according to which thestructure 4 comprises a plurality of direct-coupled optical microringresonators 210.

Optical microring resonators 210 are directly coupled to each other andto input/output waveguides 212. According to the above mentioned articleby A. Melloni et al. (“Synthesis of direct-coupled resonators bandpassfilters for WDM systems”, Journal of Lightwave Technol., Vol. 20, No. 2,February 2002, pages 296-303), the transfer function of this structureis the same as a Fabry-Perot like multiresonator structure. Thus, thepreviously explained theory holds also in this case. A microring differsfrom a Fabry-Perot cavity only for having the forward and backwardinput/outputs ports 212 all physically distinct (with the consequentadvantage that the reflected radiation is separated from the incomingradiation thereby avoiding the use of circulators or isolators). Thepump and signal radiation are supplied by an optical fiber 211 a to theinput port 212 a. If the frequencies of the pump and signal radiationare launched into the structure at two resonant frequencies of theresonators, both radiation are coupled from input waveguide 212 to thefirst ring 210 a, from the first ring 210 a to the second ring 210 b andso on towards the output ports 212 c and 212 d. The pump and signalradiation are coupled to port 212 c if N is odd while to port 212 d if Nis even, where N is the number of cascaded microrings. Port 212 b is theuncoupled output port for out of resonance input radiation.

For a cascaded microring resonator structure, the optical length of theresonator is defined as n_(eff)*πr, wherein n_(eff) is the effectiverefractive index of the material of which the structure is made of and ris the microring radius.

FIG. 10 shows another embodiment of the invention according to which thestructure 4 comprises a photonic crystal waveguide 221. The photoniccrystal waveguide 221 is obtained eliminating one or more row of defectsin a two-dimensional (2D) or three-dimensional (3D) photonic crystallattice 220. Clearly, only the frequencies within the photonic bands areallowed to propagate through the waveguide. In FIG. 10, a 2D squarelattice is shown even if different and more sophisticated lattices suchas the hexagonal lattice can be used.

Single or multiple defects (224 a to 224 z) act as partially reflectingmirror so as to form cascaded cavities 225 a to 225 z. The powerreflectivity of the mirrors depends on the dimension of the defectsthemselves. The transfer function of such a structure is the same as aFabry-Perot like multiresonator structure. Thus, the previouslyexplained theory holds also in this case. The pump and the signalradiation are supplied by an input optical fiber 222 to an input port ofthe waveguide 221. If the frequencies of the pump and signal radiationare launched into the structure at two resonant frequencies of theresonators, both radiation propagate towards an output port of thewaveguide 221. As in the Fabry-Perot like multicavity shown in FIG. 7,also in this embodiment the forward and backward input/outputs ports arenot physically distinct. Pump, signal and converted output radiation canbe collected using an optical fiber 223.

FIG. 11 shows another embodiment of the invention according to which thestructure 4 comprises a plurality of cascaded Fabry-Perot like cavitiesformed in a rare-earth doped bulk medium or a rare-earth doped waveguide230 (e.g. erbium doped). A pumping light beam tuned within the activemedium absorption band leads to a stimulated emission amplification of ageneral signal travelling through such structure. When the pump andsignal radiation involved in the FWM process fall into the active mediumgain band, the wavelength conversion process is further enhanced (asexplained, for example, by EP 0 981 189). A laser source 233 is used aspumping light beam for the active medium. Such pumping light beam ispreferably tuned far away from the reflectivity band of the mirrors ofthe cavities (for example far away from the Bragg grating band) so thatit does not experience the resonator-like propagation. The pumping lightbeam is coupled to an input optical fiber 231 a by an optical coupler232 a and an optical fiber pigtail 234 a, while a second optical coupler232 b is used to extract the pumping light beam from an output opticalfiber 231 b at the exit of the structure 4.

The Applicant notes that, although in the device 100 the maximum numberof resonators is limited by the phase mismatch (see Eq. 9), theconverted optical power P_(c,mrs) can be further increased with amultistage device 100 according to another embodiment of the invention.

The multistage device 100 comprises an input 1, an output 2, a pumplight source 3, an optical coupler 6, a plurality of cascadedmultiresonator structures 4 and a plurality of phase mismatchcompensating elements 5 interspersed with the structures 4 as shown inFIG. 1 b.

As to the features of the input 1, the output 2, the pump light source3, the optical coupler 6, each multiresonator structure 4, reference ismade to what already disclosed above.

The phase mismatch compensating elements 5 are adapted to compensate forthe phase mismatch between the pump and signal radiation at the exit ofeach structure 4.

The pump, signal and generated radiation which leave the first structureout of phase, propagate through the first phase mismatch compensatingelement without non-linear interaction, enter the second structure withthe opportune relative phase, and so on till the last structure. Thecompensating element is preferably made of a linear material so as toprevent FWM from taking place. In fact, any non-linear interaction wouldtransfer power back from the converted radiation to the pump radiationthereby reducing wavelength conversion efficiency.

According to Eq. 5, by doubling the distance z, and hence the number Nof resonators, the output converted optical power is increased of 6 dB.Thus, each additional structure 4 implies an output converted opticalpower increase of 6 dB (by suitably selecting the optical power of thepump radiation).

FIG. 12 shows the performance of a multistage device 100. With referenceto each material, each structure is equal to the one described withreference to FIG. 6. For example, in a AlGaAs multistage device a phasemismatch compensating element is preferably used after 11 resonators. Inone embodiment, a 220 resonator AlGaAs multistage device preferablycomprises 20 structures alternated with suitable designed phase mismatchcompensating elements. In FIG. 12 the point A is marked to compare theperformance of a multistage device with respect to the single stageAlGaAs device disclosed with reference to FIG. 6.

The phase mismatch compensating element can be designed taking intoaccount the following considerations.

Let Δφ be the phase mismatch accumulated between the pump and signalradiation at the exit of each structure. Using Eq. 10, the generalexpression for Δφ may be expressed as

$\begin{matrix}{{\Delta\phi} \approx \frac{\Delta\;{kL}}{t} \approx {{\beta_{2}\left( {2{\pi\Delta}\; F} \right)}^{2}\frac{L}{t}}} & (13)\end{matrix}$where β₂ is the second order dispersion coefficient (at the pumpradiation frequency) of the material the structure is made of and L isthe physical length of the structure itself (wherein L may be less thanor equal to L_(coh)). If the wavelength converter device is used in aWDM communication system, not all WDM channels experience the same phasemismatch. In fact, the higher the detuning ΔF from the pump wave, thegreater the phase mismatch Δφ. Thus, the phase mismatch compensatingelement should introduce a frequency dependent compensating termΔ{circumflex over (φ)} that satisfies the conditionΔφ+Δ{circumflex over (φ)}=2Mπ  (14)where M is an integer.

Different approaches may be followed in order to achieve a compensatingelement. For instance, a linear element made of a material having a{circumflex over (β)} opposite in sign with respect to β₂ may be used.If {circumflex over (L)} is the physical length of such a phase mismatchcompensating element, the phase term introduced is Δ{circumflex over(φ)}={circumflex over (β)}₂(2πΔF)²{circumflex over (L)}. Since the signof Δ{circumflex over (φ)} is opposite to the sign of Δφ, we will referto it as backward rephasing. Substituting the expressions of Δφ andΔ{circumflex over (φ)} into Eq. (14), we obtain the length {circumflexover (L)} of the compensating device

$\begin{matrix}{\overset{\Cap}{L} = {{\frac{\beta_{2}}{{\overset{\Cap}{\beta}}_{2}}}\frac{L}{t}}} & (15)\end{matrix}$

It should be noted that in the case of backward rephasing the physicallength {circumflex over (L)} of the phase mismatch compensating elementdoes not depend on the frequency detuning ΔF. This result derives fromthe fact that in the case of backward rephasing Eq. 14 admits a solutionfor M=0. It follows that one compensating element is enough to rephaseall the WDM channels exiting from a multiresonator structure and themultistage device may be designed to wavelength convert a plurality ofWDM optical channels.

Also, a phase mismatch compensating element made of a material having a{circumflex over (β)} with the same sign as β₂ may be used. If{circumflex over (L)} is the physical length of such a phase mismatchcompensating element, the phase term introduced is Δ{circumflex over(φ)}={circumflex over (β)}₂(2πΔF)²{circumflex over (L)}. Since in thiscase the phase mismatch compensating element adds a further frequencydependent dephasing term of the same sign as Δφ, we will refer to it asforward rephasing. When the total dephasing reaches a multiple of 2π,the pump, signal and generated radiation return to be in phase again andmay enter the following structure of the multistage device. From Eq. 14the length {circumflex over (L)} of the compensating device is achieved:

$\begin{matrix}{{\overset{\Cap}{L}\left( {\Delta\; F} \right)} = \frac{{2M\;\pi} - {{\Delta\phi}\mspace{11mu}\left( {\Delta\; F} \right)}}{\overset{\Cap}{\beta_{2}}\left( {2{\pi\Delta}\; F} \right)^{2}\overset{\Cap}{L}}} & (16)\end{matrix}$where Δφ(ΔF) is the phase mismatch at the exit of each structure,according to Eq. 13. It should be noted that in this case the length{circumflex over (L)} of the compensating device always depends on thedetuning ΔF. In fact, Eq. (14) does not admit a solution for M=0 andconsequently, once {circumflex over (L)} is chosen, forward rephasingworks only at a fixed detuning ΔF. Therefore a multistage device withforward rephasing compensating elements operates an efficient wavelengthconversion only for one WDM channel spaced ΔF from the pump radiation.If more than one WDM channel has to be compensated, backward rephasingis preferred and consequently different materials have to be used forthe structures and the compensating elements.

FIG. 13 shows an embodiment of a multistage device 100 of the inventioncomprising two structures 121 a and 121 b and a phase mismatchcompensating element 181 inserted between the two structures 121 a, 121b. The phase mismatch compensating element 181 is a dielectric opticalwaveguide. Such waveguide may be made of the same material as thestructures 121 a, 121 b (forward rephasing), or of a different material(forward or backward rephasing depending on the signs of β₂ and{circumflex over (β)}). As shown in FIG. 13, the compensating waveguide181 is wider than the waveguides of the structures. This allowsnon-linear field interactions to be prevented because the non-linearcoefficient γ decreases with an increasing waveguide effective areaA_(eff). A further reduction of the non-linear coefficient γ (and, thus,of the non-linear field interactions) may be achieved by suitablyselecting a material having a non-linear refractive index n2 lower thanthe non-linear refractive index n2 of the material of the structures 121a, 121 b.

As the widening of the waveguide transversal dimension may excite higherorder modes, the multistage device 100 of FIG. 13 preferably furthercomprises tapered input and output transition sections 182 a, 182 b.These sections are designed according to conventional techniques so asto obtain an adiabatic transition and reduced power losses.

Pump and signal radiation are supplied to the multistage device 100 froman input optical fiber 183 a and, after propagating through the firstnon linear structure 121 a, enter the phase mismatch compensatingelement 181 passing through the input tapered section 182 a. Afterpropagating through the phase mismatch compensating element 181, theyenter the second non linear structure 121 b passing through the outputtapered section 182 b. Then, pump, signal and converted radiation leavethe structure 121 b through an output optical fiber 183 b. An opticalfilter (not shown) will suppress any residual pump and signal radiation.

The lengths of the waveguide 181 and of the input and output taperedsections 182 a, 182 b are adjusted according to Eq. 15 or Eq. 16 inorder to obtain the desired relative phase between the pump and signalradiation at the beginning of the second structure 121 b.

For example, referring to the structure made of Al_(0.2)GaAs_(0.8)described above with reference to FIG. 7—whose second order dispersioncoefficient β₂ is positive (β₂˜1240 ps²/Km at 1530 nm)—the compensatingwaveguide 181 may be made of Al_(0.2)GaAs_(0.8) thereby obtaining aforward rephasing compensating element. Using Eq. 16, the length{circumflex over (L)} of such a compensating waveguide 181 is about 18mm for a detuning ΔF=2 THz. Alternatively, the compensating waveguide181 may be made of a material having a negative value of β₂, such asSiO₂ (β₂˜−25 ps²/Km at 1530 nm) so as to obtain a backward rephasingcompensating element.

Even if FIG. 18 shows a two stage device 100 with two structures 121 a,121 b and one compensating element 181, a different number of structuresand compensating elements may be employed.

FIG. 14 shows a further embodiment of a multistage device 100 of theinvention comprising four structures 121 a to 121 d arranged in parallelin a substrate 191 and three phase mismatch compensating elements 192 ato 192 c inserted in series between one structure and the other. In thisembodiment the four structures 121 a to 121 d are made ofAl_(0.2)GaAs_(0.8) while compensating elements 192 a to 192 c consisteach of a SiO₂ optical fiber (backward rephasing). Pump and signalradiation may be supplied to the device 100 through an input opticalfiber 193 a, propagate through the structure 121 a from the front facetto the back facet, enter the phase mismatch compensating fiber 192 a,hence propagate through the second structure 121 b from the back facetto the front facet and so on. Finally pump, signal and convertedradiation leave the multistage device 100 through an output opticalfiber 193 b. The length of each fiber 192 is adjusted according to Eq.15 in order to obtain the desired relative phase between the pump andsignal radiation at the beginning of each structure 121. Because of thesmall absolute value of β₂ in SiO₂ with respect to Al_(0.2)GaAs_(0.8),the length {circumflex over (L)} of each compensating fiber 192 is about66.2 cm, while the length L of each structure 121 is about 2.39 mm.

In order to reduce the length {circumflex over (L)} of the compensatingelements 192, highly dispersive SiO₂ optical fibers as well as opticalfibers made of other materials may be used. Moreover, when thestructures 121 are made of a material with a negative second orderdispersion coefficient β₂, compensating fibers 192 made of a materialhaving a positive value of β₂ may be used to perform backward rephasing.

FIG. 15 shows a further embodiment of a multistage device 100 of theinvention comprising two structures 121 a, 121 b and one phase mismatchcompensating element consisting of a dispersive plate 201 insertedbetween two structures 121 a, 121 b. An input optical waveguide 202 aand an output optical waveguide 202 b may be introduced between thedispersive plate 201 and each structure 121 a, 121 b for improving themanufacturing (lapping and cut) process. The dispersive plate 201 andthe length of each optical waveguide 202 a, 202 b are adjusted in orderto obtain the desired relative phase between the pump and signalradiation at the beginning of the second structure 121 b. Pump andsignal radiation are supplied to the device 100 through an input opticalfiber 203 a. Another optical fiber 203 b is used as output port forpump, signal and converted radiation.

A dispersive plate is a plate having high chromatic dispersion valuewith respect to the non-linear material of the structures 121 a, 121 b.Preferably, it has chromatic dispersion value ten times higher.

Of course, even if FIG. 15 shows a two stage device, a different numberof cascaded structures and dispersive plates may be employed.

The wavelength converter device of the invention can be used foravoiding a conflict of wavelengths in a node of a. WDM optical networkwherein signals at different wavelengths are routed towards other nodesof the network (S. J. B. Yoo, “Wavelength conversion technologies forWDM network applications”, Journal of Lightwave Technology, Vol. 14, No.6, June 1996, pages 955-966). In fact, in such a node it may occur thattwo signals having the same wavelength (or frequency) have to be routedtowards the same output and that a device suitable to convert thewavelength of one of the two signals into another wavelength, beforerouting the two signals towards the same node output, is required.

Moreover, since the converted radiation is the phase conjugated of thesignal radiation and falls into the symmetrical resonance mode withrespect to the pump resonance (see FIG. 2), the wavelength converterdevice of the invention can also be used as spectral inverter of asignal radiation with respect to a pump radiation.

For example, such spectral inverter can be used for compensating thechromatic dispersion of a signal or of a grid of WDM signals in anoptical communication line or system. In the case of a grid of WDMsignals, the spacing in frequency of the grid of WDM signals must besubstantially equal to the free spectral range of the wavelengthconverter device or to an integer multiple thereof.

Even if the wavelength converter device of the invention has beendisclosed with reference to the wavelength conversion through degenerateFWM process, it is clear that it can also be suitably designed toperform wavelength conversion through non-degenerate FWM process or anyanother of the above mentioned parametric processes.

Furthermore, the device of the invention, can also be used to alter(e.g. to broaden) the optical spectrum of a signal radiation propagatingthrough it according to the well known self-phase modulation (SPM) orcross-phase modulation (XPM) non-linear phenomena.

More in particular, SPM refers to the self-induced phase shiftexperienced by an optical field during its propagation in a non-linearmedium. Among other things, SPM is responsible for spectral broadeningof ultrashort pulses and the existence of optical solitons in theanomalous-dispersion regime of the non-linear medium. On the contrary,the XPM refers to the non-linear phase shift of an optical field inducedby a copropagating field at different wavelength. Among other things,XPM is responsible for spectral broadening of copropagating opticalpulses (see G. P. Agrawal, “Nonlinear fiber optics”, 2nd edition,Academic Press, 1995, page 17).

FIG. 18 shows an optical communication line 23 according to theinvention, comprising an optical cable including a transmission fibrelength 25 and a device 100 according to the invention.

The optical transmission fibre length 25 is a portion of optical fibreconventionally used for optical communications. Typically, it is aportion of single-mode optical fibre at the wavelengths of interest.

The optical communication line 23 can be used either in a long distancetransmission system or in distribution networks such as, for example,access networks.

According to the applications, the optical communication line 23 mayalso comprise an optical amplifier (not shown).

For example, the optical amplifier is of the conventional type and itcomprises a portion of erbium-doped active optical fibre and a pumpsource (for example, a laser source) for pumping the active opticalfibre at a pumping wavelength. Typical example of pumping wavelengthsfor an erbium-doped active optical fibre are about 980 and 1480 nm.

Moreover, the optical amplifier can optionally comprise more than oneoptical amplification stages.

According to an embodiment not shown, the optical communication line 23of the invention comprises a plurality of optical transmission fibrelengths 25, a plurality of optical amplifiers interposed between alength and the other of optical transmission fibre 25 and at least onewavelength converter device according to the invention (designed, forexample, for compensating at least partly the chromatic dispersion ofthe plurality of optical transmission fibre lengths 25).

In general, the device of the invention can be arranged at thebeginning, at the end or within the optical communication line 23. Forexample, in the case of application as spectral inverter, it can beinserted in the middle of the optical transmission fibre length 25 asdescribed by M. H. Chou (“Efficient wide-band and tunable midspanspectral inverter using cascaded nonlinearities in LiNbO ₃ waveguides”,IEEE Photonics Technology Letters, Vol. 12, No. 1, January 2000, pages82-84).

FIG. 19 shows an optical communication system 21 according to theinvention, comprising a transmitting station 22 for providing an opticalsignal radiation at a frequency ω_(s), a receiving station 24 forreceiving an optical signal radiation, and an optical communication line23 for transmitting the optical signal radiation.

According to a preferred embodiment, the optical communication system 21is a WDM system.

In this case, the transmitting station 22 is a conventional WDMequipment suitable to provide N optical signals having wavelengths λ1,λ2 . . . λN differing from one another (corresponding to frequenciesω_(s1), ω_(s2) . . . ω_(sN), to wavelength multiplex them and to sendthem in the optical communication line 23.

Preferably, the transmitting station 22 also comprises an optical poweramplifier (booster, not shown) for amplifying the WDM optical signalbefore sending it along the line 23 (or a certain number of opticalpower amplifiers in parallel for amplifying optical signals comprised indifferent wavelength bands).

Such wavelengths λ1, λ2 . . . N are typically selected into a range ofwavelengths comprised between about 1450 nm and 1650 nm.

Typically, the N optical signals are modulated signals, for example at100 Mbit/s, 1 Gbit/s, 2.5 Gbit/s, 10 Gbit/s, 20 Gbit/s, 40 Gbit/s or 80Gbit/s. Furthermore, the N optical signals are typically frequencyspaced by 12.5, 25, 50, 100 or 200 GHz.

The receiving station 24 comprises a conventional equipment suitable todemultiplex a WDM optical signal at its input and to send thedemultiplexed optical signals to optional further processing stages.Moreover, said receiving station 24 typically comprises also an opticalpre-amplifier (not shown) suitable to bring the WDM optical signal at asuitable power level to be received by the receiving equipment (or acertain number of optical pre-amplifiers in parallel for amplifying theoptical signals comprised in different wavelength bands).

According to the applications, the line 23 may also comprise a pluralityof conventional optical amplifiers (not shown) for amplifying a signalcoming from an upstream portion of the line, in which the signal hasattenuated during its propagation along it, and sending it in adownstream portion of the line.

Alternatively, in place of each optical amplifier, the line 23 cancomprise a number of optical amplifiers arranged in parallel foramplifying the optical signals comprised in different wavelength bands.

FIG. 20 shows an apparatus 26 for a WDM optical network node accordingto an embodiment of the invention, comprising a routing element 39, aplurality of wavelength converter devices 100 according to theinvention, N input optical fibers 30, M output optical fibers 31, Ndemultiplexing devices 27 and M multiplexing devices 28, with N, M≧1 andN equal to or different from M.

In the embodiment shown, each input optical fiber 30 is opticallyconnected to a respective 1×K1 demultiplexing device 27, thedemultiplexing devices 27 are optically connected to the input ports 32of the routing element 39, each output port 33 of the routing element 39is optically connected to a respective wavelength converter device 100,the outputs of the wavelength converter devices 100 are opticallyconnected to M K2×1 multiplexing devices 28 and each multiplexing device28 is optically connected to a respective output optical fiber 31.

Each input optical fiber 30 carries K1 wavelength multiplexed opticalsignal radiation. The K1 wavelength multiplexed optical signal radiationare wavelength demultiplexed by the demultiplexing devices 27 and sentto respective input ports 32. The optical signal radiation are thenprocessed by the routing element 39, coupled to respective output ports33, optionally wavelength converted by the devices 100 and lastlywavelength multiplexed by the multiplexing devices 28 in M outputmultiplexed optical signal radiation. The M output multiplexed opticalsignal radiation are then sent to M respective output optical fibers 31.

The apparatus shown is capable of routing any input optical signalradiation to any output optical fiber 31 with any desired wavelength(comprised within the operating wavelengths of the devices 100), thanksto the wavelength converter devices 100.

In the embodiment shown, the routing element 39 has k1×n input ports 32and k2×M output ports 33, with K1, K2>1 and K1 equal to or differentfrom K2.

According to a variant, the demultiplexing devices 27 may have a numberof output ports different from each other. Furthermore, the multiplexingdevices 28 may have a number of input ports different from each other.

The routing element may comprise a conventional optical switchingmatrix, a conventional add-drop device, a conventional cross-connect, aconventional λ-router or a combination thereof.

The optical switching matrix may comprise MEMS(micro-electro-mechanical-systems), thermo-optical, electro-optical ormagneto-optical switches which preferably are of the re-configurabletype.

The apparatus 26 may be formed by integrated optics, fiber based or MEMSbased technology.

1. A wavelength converter device for generating a converted radiation atfrequency ω_(g) through interaction between at least one signalradiation at frequency ω_(s) and at least one pump radiation atfrequency ω_(p), comprising: an input for said at least one signalradiation at frequency ω_(s); a pump light source for generating said atleast one pump radiation at frequency ω_(p); an output for taking outsaid converted radiation at frequency ω_(g); and a structure fortransmitting said signal and pump radiation, said structure including anoptical resonator comprising a non-linear material, having an opticallength of at least 40*λ/2, wherein λ is the wavelength of the pumpradiation, and resonating at the pump, signal and converted frequenciesω_(p), ω_(s) and ω_(g); said structure comprising a further opticalresonator coupled in series to said optical resonator, said furtheroptical resonator comprising a non-linear material, having an opticallength of at least 40*λ/2, wherein λ is the wavelength of the pumpradiation, and resonating at the pump, signal and converted frequenciesω_(p), ω_(s) and ω_(g), and wherein by propagating through saidstructure, the pump and signal radiation generate said convertedradiation by non-linear interaction within each of said opticalresonators.
 2. The wavelength converter device according to claim 1,wherein the converted radiation is generated by four-wave-mixing.
 3. Thewavelength converter device according to claim 1, wherein the opticalresonator and the further optical resonator each have an optical lengthlower than or equal to 7500*λ/2.
 4. The wavelength converter deviceaccording to claim 1, wherein the optical resonator and the furtheroptical resonator comprise reflectors each having a power reflectivityof at least 0.5.
 5. The wavelength converter device according to claim1, wherein the optical resonator is a Fabry-Perot like cavity bounded bytwo partially reflecting mirrors.
 6. The wavelength converter deviceaccording to claim 5, wherein the further optical resonator is aFabry-Perot like cavity bounded by two partially reflecting mirrors. 7.The wavelength converter device according to claim 1, wherein theoptical resonator is a micro-ring-like resonator.
 8. The wavelengthconverter device according to claim 7, wherein the further opticalresonator is a micro-ring-like resonator.
 9. The wavelength converterdevice according to claim 1, wherein the optical resonator is formed ina photonic crystal waveguide.
 10. The wavelength converter deviceaccording to claim 9, wherein the further optical resonator is formed ina photonic crystal waveguide.
 11. The wavelength converter deviceaccording to claim 1, further comprising an additional structure inseries to the structure.
 12. The wavelength converter device accordingto claim 11, further comprising a phase mismatch compensating elementadapted to compensate for the phase mismatch accumulated by the pump andsignal radiation along the structure.
 13. The wavelength converterdevice according to claim 12, wherein the phase mismatch compensatingelement is placed between the structure and the additional structure.14. The wavelength converter device according to claim 12, wherein thephase mismatch compensating element comprises a material having anon-linear refractive index n2 lower than the non-linear refractiveindex of the material included in the structure and the additionalstructure.
 15. The wavelength converter device according to claim 1,wherein the pump radiation frequency ω_(p) and the signal radiationfrequency ω_(s) are different.
 16. The wavelength converter deviceaccording to claim 1, wherein the optical resonator and the furtheroptical resonator are connected in series.
 17. The wavelength converterdevice according to claim 1, wherein the optical resonator and thefurther optical resonator are made of the same material.
 18. Thewavelength converter device according to claim 1, wherein the opticalresonator and the further optical resonator have the same opticallength.
 19. The wavelength converter device according to claim 1,wherein the optical resonator and the further optical resonator eachhave a free spectral range equal to or lower than about 4 THz.
 20. Thewavelength converter device according to claim 1, wherein the opticalresonator and the further optical resonator each have a free spectralrange equal to or lower than about 1000 GHz.
 21. The wavelengthconverter device according to claim 1, wherein a ratio FSR/B between afree spectral range FSR and a bandwidth B for the optical resonator andthe further optical resonator is greater than or equal to
 2. 22. Thewavelength converter device according to claim 1, wherein a ratio FSR/Bbetween a free spectral range FSR and a bandwidth B for the opticalresonator and the further optical resonator is less than or equal to100.
 23. The wavelength converter device according to claim 1, whereinthe structure further comprises a third optical resonator cascaded tothe further optical resonator, said third optical resonator comprising anon-linear material having an optical length of at least 40*λ/2, whereinλ is the wavelength of the pump radiation, and resonating at the pump,signal and converted frequencies ω_(p), ω_(s) and ω_(g), wherein bypropagating through said structure the pump and signal radiationgenerate said converted radiation by non-linear interaction within eachof said optical resonator, said further optical resonator and said thirdoptical resonator.
 24. The wavelength converter device according toclaim 1, wherein the structure comprises a number of cascaded opticalresonators less than N_(max), where N_(max) is equal to the ratiobetween the coherence length L_(coh) of the structure and the physicallength d of each of said cascaded optical resonators.
 25. A method forgenerating a radiation at frequency ω_(g) comprising: interactingthrough non-linear interaction at least one pump radiation at frequencyω_(p) with at least one signal radiation at frequency ω_(s) in astructure comprising a plurality of cascaded optical resonators eachcomprising a non-linear material resonating at the pump, signal andconverted frequencies ω_(p), ω_(s) and ω_(g), and having an opticallength of at least 40*λ/2, wherein λ is the wavelength of the pumpradiation, and wherein through said non-linear interaction the pump andsignal radiation generate said converted radiation within each of saidplurality of cascaded optical resonators.
 26. The method according toclaim 25, wherein the radiation at frequency ω_(g) is generated byfour-wave mixing.
 27. An apparatus for an optical network node,comprising: a routing element with at least one input port and aplurality of output ports for interconnecting each input port with atleast one corresponding output port; at least one wavelength converterdevice for generating a converted radiation at frequency ω_(g) throughinteraction between at least one signal radiation at frequency ω_(s) andat least one pump radiation at frequency ω_(p), comprising: an input forsaid at least one signal radiation at frequency ω_(s); a pump lightsource for generating said at least one pump radiation at frequencyω_(p); an output for taking out said converted radiation at frequencyω_(g); and a structure for transmitting said signal and pump radiation,said structure including an optical resonator comprising a non-linearmaterial, having an optical length of at least 40*λ/2, wherein λ is thewavelength of the pump radiation, and resonating at the pump, signal andconverted frequencies ω_(p), ω_(s) and ω_(g), said structure comprisinga further optical resonator coupled in series to said optical resonator,said further optical resonator comprising a non-linear material, havingan optical length of at least 40*λ/2, wherein λ is the wavelength of thepump radiation, and resonating at the pump, signal and convertedfrequencies ω_(p), ω_(s) and ω_(g); wherein by propagating through saidstructure the pump and signal radiation generate said convertedradiation by non-linear interaction within each of said opticalresonators, and said at least one wavelength converter device beingoptically coupled to one of the ports of said routing element.
 28. Theapparatus for an optical network node according to claim 27, furthercomprising an additional structure in series to the structure.
 29. Theapparatus for an optical network node according to claim 28, furthercomprising a phase mismatch compensating element adapted to compensatefor the phase mismatch accumulated by the pump and signal radiationalong the structure.
 30. An optical communication line comprising anoptical transmission path for transmitting at least one signal radiationat frequency ω_(s), and a wavelength converter device for generating aconverted radiation at frequency ω_(g) through interaction between saidat least one signal radiation at frequency ω_(s) and at least one pumpradiation at frequency ω_(p), comprising: an input for said at least onesignal radiation at frequency ω_(s); a pump light source for generatingsaid at least one pump radiation at frequency ω_(p); an output fortaking out said converted radiation at frequency ω_(g); and a structurefor transmitting said signal and pump radiation, said structureincluding an optical resonator comprising a non-linear material, havingan optical length of at least 40*λ/2, wherein λ is the wavelength of thepump radiation, and resonating at the pump, signal and convertedfrequencies ω_(p), ω_(s) and ω_(g), said structure comprising a furtheroptical resonator coupled in series to said optical resonator, saidfurther optical resonator comprising a non-linear material, having anoptical length of at least 40*λ/2, wherein λ is the wavelength of thepump radiation, and resonating at the pump, signal and convertedfrequencies ω_(p), ω_(s) and ω_(g); wherein by propagating through saidstructure the pump and signal radiation generate said convertedradiation by non-linear interaction within each of said opticalresonators, said wavelength converter device being optically coupled tosaid optical transmission path.
 31. The optical communication lineaccording to claim 30, wherein the optical transmission path is anoptical fiber length.
 32. A method for altering the optical spectrum ofat least one optical signal radiation at frequency ω_(s) comprising,interacting by non-linear interaction the optical signal radiation withan optical pump radiation at frequency ω_(p) in a structure comprising aplurality of cascaded optical resonators each comprising a non-linearmaterial, resonating at the pump, signal and converted frequenciesω_(p), ω_(s) and ω_(g), and having an optical length of at least 40*λ/2,λ being the wavelength of the pump radiation, wherein the pump andsignal radiation generate said converted radiation through saidnon-linear interaction within each of said plurality of cascaded opticalresonators.